Analysis of Energy Saving during Food Freeze Drying

Abstract

Most of researches focus on freeze drying curves of some certain food, that is, how to control temperature and pressure during freeze drying process. About why to make such freeze drying curves and why to take some kinds of pretreating and pre-freezing measures, there are less researches dealing with. In this paper, based on equations of heat transfer and mass transfer, factors affecting freeze drying rate are analyzed systematically, which provides theoretic basis for how to do pretreating and pre-freezing and how to control temperature and pressure during freeze drying so as to shorten drying time. And the results are verified by two freeze drying figures. The results show (1) in order to shorten drying time and save energy, food should be cut into proper size and can be pretreated with ultrasound or other technologies which make food structure porous. (2) From the point of raising freeze drying rate, slow pre-freeze is better than quick pre-freeze, especially at the late stage of freeze drying which is controlled by mass transfer. (3) at the earlier stage of drying (generally controlled by heat transfer), temperature of food’s sublimation interface should be relatively low and absolute pressure of drying box relatively high so that there is enough heat transfer driving force and heat transfer coefficient. (4) at the late stage (always controlled by mass transfer), temperature of food’s sublimation interface should be relatively high, and absolute pressure of drying box relatively low to ensure there are enough mass transfer driving force and water vapor diffusion coefficient.